Did you miss the part where Sylvanas slaughters ALL of the Desolate Council, including those who were willingly returning to Forsaken lines when the return was sounded?
Sylvanas 'cares' like an abusive spouse cares that you belong to them. The scene where the Forsaken who's relatives decide not to see them return perfectly illustrates this. She HOPES the Gathering is a failure because it reinforces their dependence on her - this isn't interpretation, this is made explicit several times - and she doubles down on the, "See? You only have me. Everybody else will hurt you. You'd better stick with me" propaganda. We also discover that she runs a campaign that actively tries to make the Forsaken forget about their past lives - books are banned that aren't new, pro-Sylvanas literature - so they can double down on their dependence.
Even though they came about literally because they thought they were doing Sylvanas a favour, her first thought about learning of them is "I want to kill them all". Even Nathanos of all people asks what the hell she's doing when she orders them all killed.
She's a total control freak and insane.